There are a number of applications where determining the time-of-flight (TOF) of an acoustic signal is required. These applications include ultrasonic level detectors and ultrasonic flow meters. In general, a system for determining TOF of an acoustic signal can take two forms: pitch-catch and pulse-echo. FIG. 1A illustrates an example of a pitch-catch TOF measurement system, and FIG. 1B shows an example of a pulse-echo TOF measurement system. In either configuration, a device 110 (e.g., an acoustic transducer) transmits an acoustic signal at a predetermined time and the acoustic signal is then sensed some time later. In a pitch-catch system shown in FIG. 1A, the acoustic signal is sensed by a second, separate, receive (RX) device 120 (e.g., another acoustic transducer). In the pulse-echo system shown in FIG. 1B, the acoustic signal is sensed with the same device 110 that transmitted the signal—i.e., the device 110 is both the TX device and the RX device. The time it takes the signal to traverse from the TX device to the RX device is the time-of-flight of the signal.
Achieving a required degree of accuracy in the time-of-flight measurement of an acoustic signal is both critical and difficult.
For one thing, to measure the time-of-flight of the acoustic signal the RX device requires accurate information identifying the time when the acoustic signal was transmitted by the TX device. This presents limitations, particularly in the case of a pitch-catch system. If a copy of the electrical signal driving the acoustic transducer in the TX device is used for the receive device's reference, then some means must be provided to communicate a sample of that electrical signal from the TX device to the RX device. Furthermore, a sample the electrical signal driving the acoustic transducer in the TX device does not reflect the delay in the acoustic transducer.
What is needed, therefore, is an accurate method of estimating the time-of-flight of an acoustic (e.g., an ultrasound) signal. What is also needed is a system which can accurately estimate the time-of-flight of an acoustic signal.